Such a heating device for heating a sensor element used in a gas sensor for exhaust gas analysis of internal combustion engines, is referred to in German Published Patent Application No. 198 34 276, for example. The sensor element includes solid electrolyte layers, and electrodes, as well as a heating device having a layered configuration. The heating device is arranged on a solid electrolyte layer or between one solid electrolyte layer and another solid electrolyte layer. The heating device includes a heating element in the form of an electrical resistor layer, for example, as well as an insulation in which the heating element is embedded. The insulation has a uniform material composition and a uniform structure, in particular with regard to the porosity. The insulation serves the purpose of electrically insulating the heating element against the solid electrolyte layers and the electrodes, thus with regard to both electron and ion conduction, so that the operation of the heating device does not affect the function of the sensor element.
The heating device is manufactured by applying a lower layer of insulation, the heating element, and an upper layer of insulation, using thin-layer or thick-layer techniques, to a green foil, i.e., an unsintered ceramic foil. Subsequently, the green foil with the heating device printed on it and other green foils, with electrodes optionally printed on them, for example, are laminated and sintered.
The insulation frequently has a porous configuration. The porosity of the insulation is achieved by adding a pore former before sintering. The pore former burns during sintering and thus creates a porous structure. The porosity is adjusted through the added amount of pore former, for example glass carbon.
An insulation having a higher porosity may be disadvantageous, because leak currents may occur during operation. These leak currents develop due to contaminations within the insulation, for example by volatilization of a metallic component of the heating element, platinum for example, or due to moisture in the heating device. The higher the porosity of the insulation, the easier contaminants may penetrate the insulation and deposit in the pores of the insulation, and the higher the leak current.
On the other hand, an insulation having a lower porosity has the disadvantage that with a decreasing porosity the elasticity of the insulation decreases, resulting in increased susceptibility to cracking. This may result in an interruption of the heating element's operation. The risk of crack formation is also given if moisture penetrates into the heating device, for example, via a contact of the heating element. The lower the porosity of the insulation, the less volume is available to absorb the gas developing during heating-up and the higher the susceptibility to cracking.
In the heating devices of other prior systems including only one insulation with a uniform porosity, either the discussed disadvantages of an insulation having a higher porosity, or the disadvantages of an insulation having a lower porosity occur.